Stripline termination device having a resistor that is shorter than one quarter wavelength



Nov. 21, 1967 M. STEIDLITZ 3,354,412

STRIPLINE TERMINATION DEVICE HAVING A RESISTOR THAT IS SHORTER THAN ONEQUARTER WAVELENGTH Filed Nov. 1, 1965 b m I R & \I R O m M m S M) k m.2",", w m P 9 w. M G G 0 F l a U I I BY MARK'S United States Patent3,354,412 STRIPLINE TERMINATION DEVICE HAVING A RESISTOR THAT IS SHORTERTHAN ONE QUARTER WAVELENGTH Mark Stcidlitz, Qherry Hill, N.J., assignorto EMC Technology, Inc., Philadelphia, Pa., a corporation ofPennsylvania Filed Nov. 1, 1965, Ser. No. 505,779 7 Claims. (Cl. 333-22)This invention relates to a resistive device for microwave circuitry andparticularly to a termination device for a microwave striplinetransmission line. Most particularly, the present invention relates to atermination device for stripline microwave circuitry having broad bandcharacteristics. The present invention also has applications as ageneral resistive device in stripline circuitry such as, for example,T-pad and vr-pad attenuators, as well as power splitting resistors andso forth.

In microwave circuitry it is necessary for any resistors interposedtherein to match the resistance in the line with the nominal impedanceof the line, the nominal impedance being determined by the physicaldimensions of the line, the ground plane spacing, and the dielectricconstant of the material on which the transmission line is mounted. Ifthere is any serious mismatch between the resistance in the line and theimpedance in the line, there normally results a reflected energy wavewhich destroys the transmitting characteristics of the line. Thus, thematch of resistance and impedance is necessary to obtain a low voltagestanding wave ratio (VSWR) across a broad band. Termination resistorsand other microwave resistors in coaxial microwave circuitry have beendesigned to yield low VSWR across a broad band. However, for striplinemicrowave circuitry this has not been the case prior to the presentinvention. As a matter of fact, it is often necessary to interpose anadaptor into stripline circuitry, converting it to coaxial circuitry inorder to interpose a properly matched resistor. This is undesirable froma labor and circuit reliability viewpoint as well as from spaceconsiderations and tends to destroy the advantages of striplinecircuitry.

In accordance with the present invention, the resistive device isconstructed so that the line impedance changes along the resistor pathin order that it always be equal to the resistance to ground from thatpoint on the resistor path. This resulting match between resistor andline yields a very low VSWR across a broad band, such as from DC tomicrowave frequencies in excess of go. The desirable matchingcharacteristic of resistors made in accordance with the presentinvention are achieved by providing a conductive body with asubstantially wedge shaped cavity extending in from the side wall of theconductive body and mounting a flat resistor symmetrically within thecavity, which resistor is to be connected with the stripline circuitry.The resistor is conductively connected to the metal body at the apex ofthe wedge shaped notch.

It is therefore the main object of the present invention to provide aresistive device for use in stripline microwave circuitry, which deviceprovides a continuous match between the resistance from the resistor toground at any point along its length and the line impedance at thatpoint.

Another object of the present invention is the provision of a new andimproved resistive device for stripline circuitry, which device is madeof a conductive body having two spaced apart ground planes which areconnected by a peripheral side wall, the conductive device having asymmetrical substantially wedge shaped cavity extending therein from theperipheral side wall with a resistor disposed symmetrically within thewedge shaped cavity and being connected to the conductive body of thedevice at the apex of the wedge.

Still another object of the present invention is the provision of a newand improved termination device for stripline microwave circuitry.

Yet a further object of the present invention is the provision of a newand improved resistive attenuator for stripline microwave circuitry.

The above and other objects, characteristics and features of the presentinvention will be more fully understood from the following descriptiontaken in connection with the accompanying illustrative drawing.

In the drawing:

FIG. 1 is a perspective view of a stripline termination device embodyingthe present invention;

FIG. 2 is a top plan view thereof;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is a view similar to FIG. 3 showing a modified form of thepresent invention;

FIG. 5 is another view similar to FIG. 3 showing an other modified formof the present invention;

FIG. 6 is a perspective view of a T-pad attenuator for striplinemicrowave circuitry embodying the present invention;

FIG. 7 is a top plan view of the device of FIG. 6;

FIG. 8 is a sectional view taken along the line 8-8 of FIG. 7; and

FIG. 9 is a sectional view taken along the line 9-9 of FIG. 7.

Referring now to the drawing in detail, and especially to FIGS. 1 to 3thereof, a termination device for incorporation in stripline microwavecircuitry is shown in FIGS. 1 to 3 and is generally designated by thereference numeral 10. Termination device 10 includes a conductive body12 having upper and lower surfaces 14 and 16, respectively, and aperipheral side wall 18. Preferably, the conductive body 12 is made outof a suitable conductive metal such as brass, although copper, aluminum,and other metallic conductors could be employed. While the surfaces 14and 16 are here shown as planar, which is desirable from convenienceviewpoint, other types of surfaces could be employed without departingfrom the present invention. However, the spacing of surfaces 14 and 16should not be less than the ground plane spac- The metal body 12 isprovided with a substantially wedge shaped cavity 20 which extends infrom the peripheral side wall 18 and which is symmetrical about acentral plane extending from the apex of the wedge shaped cavity. As isshown, the maximum width of the wedge shaped cavity 20 is at theperipheral side wall 18 with the surfaces 22 and 24 defining the wedgeshaped cavity being in converging relation as they extend in from theperiphery of the metallic body 12. At the apex of the wedge shapedcavity 20 there is a substantially planar slot 30 adapted to receive aflat resistor 32 to be described in greater detail hereinafter. Forconvenience of fabrication, cavity 20 and planar slot 30 extend the 3full width of the metal body 12, as best seen in FIGS. 1 and 2.

As previously noted, a fiat resistor 32 having a width equal to thewidth of the stripline circuitry into which it is adapted to beconnected and a resistance substantially equal to the line impedance isdisposed within the wedge shaped cavity 29 and is connected to the metalbody 12 by disposition in the planar slot 30 and by added connectivemeans such as solder or the like. The resistor 32 extends outwardly fromthe slot 30 toward the peripheral side wall 18 of the body 12 asubstantial distance to be defined hereinafter but does not extend allthe way to the periphery 18 of the metallic body 12. The resistor 32 isdisposed along the plane of symmetry of the wedge shaped cavity 20.Connected to the resistor adjacent the end closest to the peripheralwall 18 of the body 1211 is a conductive tab 34 which is adapted to bedirectly connected into the stripline circuitry. Preferably the tab 34is tinned copper which is readily solderable to a surface of theresistor 32 and to the stripline circuitry on a circuit board not seenherein.

In order to hold the resistor 32 and the tab 34 in their relationshipsalong the plane of symmetry of the wedge shaped cavity 20, it ispreferred that the cavity is filled with a suitable dielectric material,preferably one which has a dielectric constant which matches thedielectric constant of the circuit board upon which the striplinecircuitry is mounted. That is to say that if the stripline circuitry ismounted on epoxy bonded fiberglass, then the back fill material 36 iseither epoxy or glass fiber filled epoxy. However, other dielectricmaterials can provide a suitable match, and I have successfully filledcavity 20 with glass ceramic, polypropylene, other plastics and othersuitable dielectric materials. Moreover, in certain applications, thecavity can be left unfilled, in which case air will serve as thedielectric material. In addition, the cavity may be filled with acombination of diiferent dielectric materials to serve simultaneouslyseveral functions. For example, a portion of a cavity may be filled withair to match the air dielectric of the stripline, another portion withalumina to match the resistor substrate, and a third portion of berylliafor its dielectric and heat conducting properties.

It is obvious that the value of the resistor 32 must be carefullycontrolled. The most convenient way to provide a closely controlledresistor 32 is to make the resistor as a resistive film on a ceramicbase by utilizing vapor deposition techniques. Moreover, such afabricated resistor gives easier handling properties for fabricating thetermination device 10. When using a vapor deposited resistor, it isgenerally preferable to take a flat ceramic base, deposite a layer ofresistive material such as nickel chromium alloy onto a surface of thatbase for a sufii-' ciently long period to provide .a resistor of desiredresistance value.

The effective length of the resistor 32 is of great importance in orderto render the device effective. By effective length, I mean that portionwherein the resistor alone transmits energy. Thus, the effective lengthof resistor 32 is the length between the right hand end of the slot 30as viewed in FIG. 3 and the left hand end of the conductive tab 34 asviewed in said FIG. 3, that length being the length in between which theresistor actually functions as the resistive load, the remainder of theresistor 32 being employed merely to provide connecting areas to theslot 30 and the tab 34. In order to have a .terrninationdevice ofsuitable quality it is necessary for the elfective length of theresistor 32 to be less than one-quarter of the shortest wave length tobe encountered in the operating frequency range of the cir-. cuitry intowhich the device will be incorporated. Preferably, the effective lengthof the resistor 32 should be less than one-tenth of the shortest wavelength to be encountered and most preferably less than one-twentieththereof.

While a true wedge shaped cavity 20 as shown in FIG. 3 gives excellentresults for the termination device 10, other cavity configurationsapproximating a wedge can be employed within the scope of the presentinvention. Thus, in FIG. 4 the device 10 is modified only with respectto the shape of the cavity 20. The cavity 20', although approximating awedge shape, is not a true wedge as is the cavity 20 of FIG. 3.Specifically, the upper surface 22' and the lower surface 24' of thecavity 20 are slightly arcuate rather than planar, as was true in theFIG. 3 construction. Such a modification may at times be useful forproviding corrections for ground plane spacing as when taking intoaccount line impedance errors due to fringing capacitance.

Referring now to FIG. 5, another modified form of a substantially wedgeshaped cavity is shown in the termination device 10". This cavity,herein designated 20", is wedge shaped but only along the eifectivelength of the resistor 32 as above described. The remainder of thecavity is substantially rectangular in cross section. Thus, the upperwall 22" is made of two portions 22"a and 22"b, the first of which issubstantially parallel to the upper surface 14 of the body 12 and thesecond of which, the portion. 22"b, extends at an angle away from thatsurface. Likewise, the lower surface 24" has two portions 24"a and24"11, the first of which is substantially parallel to the surface 16and the second of which extends up wardly at an angle therefrom. It isonly the portions 22"b and 24"b which provide the actual wedge shape ofcavity efiect, the remainder of the cavity being substantiallyrectangular as previously stated. In constructing such a cavity 20-, itis desirable for the portions 22"a and 24"a to be as close to the. upperand lower surfaces 14 and 16, respectively, as manufacturing procedureswill allow.

Other modified wedge shaped cavities 20 will readily suggest themselvesto those skilled in the art. However, irrespective of the preciseconfiguration of the cavities 20, it is necessary for the cavity to havea general substantially wedge shape in order to conform to the teachingof the present invention.

As previously mentioned in this description, the principle ofmaintaining a matched impedance between line and resistor to groundalong the entire path of a resistor stripline microwave circuit can beemployed in resistive devices other than termination devices. An exampleof another application of this principle is shown in FIGS. 6 through 9,wherein a T-pad attenuator for stripline microwave circuitry isillustrated. A T-pad attenuator is normally connected into a striplinecircuit to limit the amount of energy being conducted along the circuit.While an ordinary resistor could perform such a function, it will beobvious that the interposition of an ordinary resistor in such a circuitwould change the line impedance of the circuit and thereby be completelyunacceptable. Thus, what is normally done is to introduce into thecircuit an attenuator which is adapted to shunt excess energy off toground without significantly changing the impedance of the striplinecircuit. In prior art devices, while at relatively low frequency levelsa T-pad attenuator would function well, at higher frequency, due to amismatch between line and ground impedance along the length of theattenuator, there would be significant reflected energy, which wouldgive rise to a standing wave and hence destroy the operating quality ofthe circuit. In the present ably metallic and may be made of anysuitable conductive metal such as brass, aluminum, copper and the like.The conductive body 112 has an upper ground surface 114, a lower groundsurface 116 and a peripheral side wall 118, here shown to be square inconfiguration. Metallic body 112 has within it a cavity 120 having astraight through cavity portion 121 and a dead ended portion 123perpendicular thereto. Disposed within the straight through cavityportion 121 is a flat series resistor 125 which is preferablyconstructed in the same manner as resistor 32 above described. That is,the resistor 125 has a ceramic base on which is vapor deposited aconductive material the thickness of which is carefully controlled toyield desired resistance. The width is substantially equal to that ofthe external stripline circuitry. As shown herein, the fiat seriesresistor 125 is actually made of two flat abutting resistor portions125a and 125b which are joined together by a strip of contact materialsuch as copper 127. Each end of resistor 125 is connected inelectrically conducting relation to a conductive tab 134 which ispreferably tinned to facilitate its connection with the stripline of themicrowave circuitry.

As may best be seen in FIG. 8, the cavity portion 121 of cavity 120 ismade up of two Wedge shaped portions 121a and 121b with the seriesresistor portions 125a and 1251) disposed respectively therein and alongthe plane of symmetry of the two opposed wedges 121a and 121b. It willbe noted that the form of the wedge shaped cavities 121a and 121b issimilar to the form of cavity 20" in FIG. 5, although other forms ofsubstantially wedge shaped cavities might be employed in accordance withthe present invention. This wedge shaped configuration especially alongthe effective lengths of resistor portions 125a and 125b yields aconstant match between the line impedance and the impedance fromresistor to ground along the entire length of the resistor portions. Inaddition, a flat shunt resistor is provided to shunt excess energy fromthe resistor 125 to the grounded conductive body 112 of the attenuationdevice. This shunt resistor is designated by the reference numeral 130and is of substantially the same width as the stripline circuitry andextends from the series resistor portion 125 to electrical conductiveconnection with the conductive body 112 at the apex of the wedge shapedcavity portion 123. It will be noted that the shunt resistor 130 is alsodisposed along the plane of symmetry of the cavity 120 and especially ofthe cavity portion 123 thereof. Electrical connection to the straightthrough series resistor portion 125 is shown to be effected by theconductive tab 127. With the shunt resistor 130 disposed along the lineof symmetry of wedge shaped cavity 123 as above described, there willalways be an impedance match between the line impedance and theimpedance to ground along the shunt resistor 130 to thereby greatlyreduce any tendency to set up a standing wave.

As may be seen in FIGS. 7 and 8, the entire portion of cavity 120unoccupied by the resistors or connective members is filled with adielectric material of the same type as the dielectric material 36 abovedescribed. This dielectric material is herein designated by thereference numeral 136.

While the manner of fabricating a cavity 120 may be left to anyoneskilled in the art, as shown herein the cavity portion 121 isconstructed by machining out from the two sides thereof the two separatewedge shaped portions 121a and 121b, and a portion of this machining maybe accomplished by coming in through the surface extending parallel tothe line of extent of said cavity 121, namely through the surface 135(FIG. 6). Moreover, the opening 137 in surface 135 may be enlarged asillustrated to facilitate the access for machining the wedge shapedcavity portion 123 extending perpendicular to the cavity portion 121above discussed.

It will be obvious that the cavity 120 could be modified in accordancewith the teachings of FIGS. 3 and 4 above and need not be constructed inaccordance with FIG. 5 above. That is to say, the tapering of the wedgeshaped portions 121a and 121k could commence at the left and right handends of the cavity portion 121 as viewed in FIG. 8 and could join at anapex which is central of the cavity rather than have the planar ornontapering portions in the area of non-efiective resistance.Alternatively, the tapering surfaces need not be flat but could bearcuate as suggested above with regard to FIG. 4.

It is also obvious that having had the above invention relating tostripline resistive devices described in connection with a terminationdevice and a T-pad attenuation device, other stripline resistive deviceswherein the matching of line impedance along the resistor with theimpedance to ground is important may be designed within the scope of thepresent invention. For example, power splitting resistors and 11' padattenuators could readily be de signed in accordance with the teachingsof the present invention.

While I have herein shown and described several forms of the presentinvention and have suggested modifications therein, other changes andmodifications may be made therein within the scope of the appendedclaims without departing from the spirit and scope of this invention.

What I claim is:

1. A resistive device for a line of stripline microwave circuitry,comprising a conductive body having top and bottom surfaces and aperipheral side wall, said conductive body having a substantially wedgeshaped cavity extending in from said peripheral side wall, said cavitybeing of maximum thickness at said peripheral side wall and beingsymmetrical with respect to a plane including the apex of said wedgeshaped cavity, a fiat resistor disposed wholly within said cavity alongsaid plane and being connected at one end to said conductive body inelectrically conducting relation at the apex of said cavity, and meansfor connecting the other end of said resistor in electrically conductingrelationship with stripline circuitry external of said device, theeffective length of said resistor being shorter than one-quarter of theshortest wave length in the operating frequency range thereof.

2. The device of claim 1, wherein said device is a termination devicewhich further comprises dielectric material in said cavity for holdingsaid resistor in said cavity along said plane. I

3. The device of claim 2 wherein the effective length of said resistoris shorter than one-tenth of the shortest wave length in the operatingfrequency range thereof.

4. The device of claim 2, wherein the resistance of said resistor issubstantially equal to the impedance of said line.

5. An attenuation device for stripline microwave circuitry, comprising aconductive body having upper and lower surfaces and a peripheral sidewall, said conductive body having a cavity therein, said cavity having afirst longitudinal portion intersecting said peripheral side wall at twodistinct areas, said first cavity portion having two substantially wedgeshaped parts with the maximum widths thereof at said areas ofintersection with said peripheral side wall, said cavity having a secondportion perpendicular to and in intersecting relation with the first andhaving one end terminating within said conductive body, the part of saidsecond portion extending between said first portion and said one endbeing substantially wedge shaped, said cavity being symmetrical about aplane including the apexes of said two wedge shaped cavities, a firstflat resistor disposed within said first cavity portion along said planeof symmetry, a second flat resistor disposed within said second cavityportion along said plane of symmetry, said second resistor having oneend connected to the apex of the wedge shaped part of said second cavityportion in electrically conductive relation with said conductive body,means for connecting the other end of said second flat resistor inelectrically conducting relation with the center of said first resistor,a

fiat electrically conductive member connected to one end of said firstresistor and extending out of said conductive body through one end ofsaid first cavity portion, another fiat electrically conductive memberconnected to the other end of said first resistor and extending out ofsaid conductive body through the other end of said first cavity portion,the effective length of each of said two resistors being shorter thanone-quarter of the shortest Wave length in the operating frequency rangethereof.

6. The attenuation device of claim 5, wherein said first flat resistorincludes two separate resistive parts, and conductive means forelectrically connecting said two parts, said last mentioned meansfurther being for connecting said first resistor to said secondresistor.

7. The attenuation device of claim 6, wherein said two resistor partsand said second resistor all have effective lengths shorter thanone-tenth of the shortest wave length in the operating frequency rangeof said device.

References Cited UNITED STATES PATENTS 3,174,123 3/1965 Frederico .33322X 3,213,392 10/1965 Hedberg 33322 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

P.L. GENSLER, Assistant Examiner.

1. A RESISTIVE DEVICE FOR A LINE OF STRIPLINE MICROWAVE CIRCUITRY,COMPRISING A CONDUCTIVE BODY HAVING TOP AND BOTTOM SURFACES AND APERIPHERAL SIDE WALL, SAID CONDUCTIVE BODY HAVING A SUBSTANTIALLY WEDGESHAPED CAVITY EXTENDING IN FROM SAID PERIPHERAL SIDE WALL, SAID CAVITYBEING OF MAXIMUM THICKNESS AT SAID PERIPHERAL SIDE WALL AND BEINGSYMMETRICAL WITH RESPECT TO A PLANE INCLUDING THE APEX OF SAID WEDGESHAPED CAVITY, A FLAT RESISTOR DISPOSED WHOLLY WITHIN SAID CAVITY ALONGSAID PLANE AND BEING CONNECTED AT ONE END TO SAID CONDUCTIVE BODY INELECTRICALLY CONDUCTING RELATION AT THE APEX OF SAID CAVITY, AND MEANSFOR CONNECTING THE OTHER END OF SAID RESISTOR IN ELECTRICALLY CONDUCTINGRELATIONSHIP WITH STRIPLINE CIRCUITRY EXTERNAL OF SAID DEVICE, THEEFFECTIVE LENGTH OF SAID RESISTOR BEING SHORTER THAN ONE-QUATER OF THESHORTEST WAVE LENGTH IN THE OPERATING FREQUENCY RANGE THEREOF.